Event Description
During cruise flight, the pilot noticed an uncharacteristic vibration from the engine. Diverting to a nearby airport, the pilot entered a downwind leg for the intended runway when the engine stopped producing power. The pilot then turned directly toward runway 31. After completing the turn to final approach, the pilot landed on the intended runway, describing the touchdown as "very hard." Following the landing, the pilot's seat was found to have collapsed as a result of the impact.
Engine Examination
Inspection of the aircraft's engine revealed a fracture in the number four cylinder head. The fracture extended entirely through the cylinder head, originating from the top sparkplug hole and running around the circumference to the bottom sparkplug hole. A metallurgical examination determined that the cylinder head failed due to fatigue. Multiple fatigue crack initiation sites were identified, most notably a series of cracks that began at each of the two sparkplug holes.
Maintenance History
Maintenance records indicated that the failed cylinder had been previously repaired for valve pitting and returned to service. At the time of the repair, the engine had accumulated 6,689 total hours and 726 hours since a major overhaul. The total time since manufacture for the cylinder itself could not be determined. At the time of the accident, the engine total time was approximately 7,260 hours, with 1,319 hours since the last major overhaul.
Conclusion
The metallurgical examination concluded that the cylinder head fractured due to fatigue, with multiple initiation sites at the sparkplug holes.